The United States government should never be tempted to embrace a sole purpose declaration in regard to its nuclear weapons policy. As discussed recently by Rod Lyon, (here and here) and Crispin Rovere, such a declaration states that the ‘sole purpose of possessing nuclear weapons is to deter the use of such weapons against one’s own state, and that of one’s allies’.

There’s a pressing need to reassure allies about the credibility of US extended nuclear deterrence security guarantees at a more dangerous time in the world. Those guarantees serve two key purposes. First, they reduce the potential for overwhelming threats to US allies, including Australia (see the 2013 Defence White Paper, para 3.41). Such threats would include nuclear attack, but could also include large-scale conventional or even biological attack. Drawing a degree of strategic equivalence between a nuclear attack and large-scale non-nuclear attacks reinforces deterrence and makes such contingencies less likely to occur.

Second, extended nuclear deterrence security guarantees reduce the risk that states will need to consider acquiring their own independent means of deterrence, including their own nuclear weapons, and potentially generating proliferation cascades that could dramatically destabilise parts of the globe. How would China react to Japanese acquisition of nuclear weapons? How would South Korea?

A sole purpose declaration would provide added caveats on extended nuclear deterrence to vital allies—weakening those security assurances. I doubt that’s a good move, with a powerful and assertive China and a revanchist Russia both challenging US strategic interests, the international rules-based order more broadly, and their neighbours’ security interests. Oh, and by the way, both China and Russia are modernising their nuclear forces as the US continues to let its nuclear weapons age out, with no guarantees of adequate funding for modernisation in the future.

In its haste to achieve the ethically well-meaning, but strategically dubious objective of nuclear abolition in a more dangerous and conflict-prone world, the International Commission on Nuclear Non-Proliferation and Disarmament (ICNND) promoted the idea of a sole-purpose declaration as an interim step towards a ‘No First Use’ declaration, and as a means towards delegitimising nuclear weapons in general. The Commission believed that delegitimising nuclear weapons would make them easier to abolish. But the notion of a sole-purpose declaration ignores the reality that states choose to acquire nuclear weapons for valid strategic purposes—including deterrence, compellence, assurance, or even bargaining or national prestige. Those weapons have strategic and operational value in the eyes of Moscow, Beijing and others. The same case can be (and is) made in Western capitals—though there the democratic process gives more opportunities for pro-abolitionist voices to challenge the utility of nuclear weapons.

A number of practical problems would, in any case, compound such a declaration. What happens, for example, if the US makes a sole purpose declaration, but Russia and China don’t follow suit? China already maintains a declaratory ‘no first use’ doctrine, but the latest US Department of Defence report to Congress on Chinese military power (PDF) notes ambiguity over the conditions under which that policy would apply. It’s doubtful that Russia would entertain such an idea, given its tense relations with what it perceives to be an expanding NATO. So is it only the US, UK and France that are to tie their hands? If Russia and China were to follow suit, how much credence could be placed upon such a declaration? How is such a declaration to be monitored and verified? And how robust would any declaration prove to be in a crisis? After all, what has been declared can be undeclared. Such declarations are, by their nature, freighted with uncertainties, and could in fact generate escalatory dynamics and make crisis management more difficult.

Such a declaration does nothing to delegitimise nuclear weapons in the eyes of states who really want them. Is North Korea going to buy into this…or Iran? The US and its allies need to think about reinforcing the credibility and effectiveness of nuclear deterrence to counter old and new threats. A sole-purpose declaration weakens deterrence and is a bad idea.

Malcolm Davis is assistant professor in International Relations and post-doctoral research fellow in China-Western Relations at Bond University.